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Full text of "Handbook Of Chemical Engineering - I"

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SECTION  V
CRUSHING AND GRINDING
BY H. A. MEGRAW*
The size reduction of materials of various kinds forms an important step in the technical operation of many industries, so that, in the course of time, a particular system of machinery has been designed to perform the work simply and economically. The principles of division and subdivision are essentially the same for all solids, whether they may be rocks, minerals or composites. Reduction machinery, however, has been designed particularly for operating upon friable or non-elastic materials. Substances that are highly elastic or plastic, or which become plastic under small temperature rises, are not suitable for subdivision in machines of the ordinary type. They are to be handled by machinery coming under the classification of cutting, rending or abrasion machines. Such materials as rubber, wood, gums, rosins and some chemical compounds are not to be broken or pulverized successfully in the usual reduction machines, but must be turned over to machinery of a different type.
At the same time there are materials that lie on the border line between friable and elastic substances that can be reduced in size by appliances embodying the principles of standard crushing equipment, though somewhat modified for this special service. Thus many materials can be ground satisfactorily in the old-fashioned buhr-stone mill which could not be handled so well with more modern machinery. The buhr mill employs roughened hard stone faces moving one upon the other, the effect being to rend or tear the material to be ground, as well as to abrade it.
Bulir stones would not be considered satisfactory appliances for ordinary breaking or pulverizing, due to the high rate of wear and the consequent expense of maintenance, as well as small capacity, but for many special problems they provide the only solution. Among these are many compounds beside the exceptional ones above listed.
Industrial Rock Reduction.—There are many industries in which the breaking, grinding and pulverizing of rock, stone or ore is an essential feature. Among these industries may be mentioned quarrying, cement-making, mining, glass-making, lime manufacture, pottery, sand and gravel production and many others. They involve the reduction in size of rock from one point to another, the character or size of the product depending entirely upon the service for which it is intended. Rock reduction alone in the United States amounts to many millions of tons' yearly for all purposes. In view of the extent of this operation, it is somewhat surprising that more attention is not given to the principles, mechanics and costs of such work.
The several stages of rock-breaking may be divided generally into three, which for the purpose of this paper may be called breaking, crushing, and grinding.
Rock-breaking may be limited to the primary operation of reducing by machinery
1 Consulting engineer, Baltimore, Md.
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